The present invention relates to an apparatus and method by which viscous liquid compositions may be coated on to a web such as a continuously moving web of material, as in the manufacture of photographic material such as films, photographic papers, magnetic recording tapes, adhesive tapes, etc.
Bead coating is a method of coating used extensively in the manufacture of photographic material and products. In this method a liquid bridge known as the bead is formed between a coating die and a web to be coated. The basic technology is well established and is discussed in, for example, U.S. Pat. Nos. 2,681,294 and 2,761,791. The latter relates specifically to multi-layer coating in which two or more layers are simultaneously applied to a continuously moving web of material in the manufacture of photographic materials. Typically, multi-layer flow of a coating liquid on an inclined plane of the die is used. The coating liquid flows towards the web to be coated and contacts the web at an impingement point. There are a number of limitations to the maximum speed of the bead coating method and these are discussed in, for example, the book entitled xe2x80x9cLiquid Film Coatingxe2x80x9d, ed. S. F. Kistler and P. M. Schweizer, Pub. Chapman Hall. A fundamental limit to the coating speed in most coating operations is the incorporation of macroscopic bubbles of air between the web and the coating solution, known as air entrainment.
Various methods have been proposed to postpone air entrainment to higher speeds of coating i.e. to increase the threshold coating speed above which air entrainment occurs. In many of these methods, advantage is taken of the understanding that lower viscosities for the layer that wets the web (the wetting layer) enables the threshold speed to be increased. However, having a low viscosity layer adjacent to the inclined plane of the coating die can endanger the stability of the multi-layer flow. More importantly, the bead stability is reduced as the viscosity of the wetting layer is reduced. To address these problems a shear thinning lowermost layer may be used, as disclosed in U.S. Pat. No. 4,113,903, which issued in the name of E. J. Choinski on Sep. 12, 1978. In this patent, a system is provided in which the viscosity in the majority of the bead is high while the viscosity at very high shear near the wetting line is low. In this way the conflicting requirements of low viscosity for high-speed wetting and high viscosity for flow on the inclined plane of the coating die and in the bead can be satisfied simultaneously.
Another method to increase the threshold speed above which air entrainment occurs employs the use of an electrostatic field as disclosed in International Patent Application No. WO 89/05477 in the name of Eastman Kodak Company. In addition to increasing the threshold coating speed, an electrostatic field applied at the impingement point has a beneficial effect of stabilizing the bead in a similar way to that of a vacuum or reduced pressure region provided beneath the bead between the coating die and the web. It is well known that providing a charge on the web surfaces and grounding both the coating roller and the coating liquid may generate the electrostatic field. This is disclosed, for example, in International Patent Application No. WO 89/05477 or U.S. Pat. No. 4,835,004 in the name of Kawanishi. It is also possible to bias the coating roller whilst maintaining the coating liquid at ground potential, as disclosed in U.S. Pat. No. 3,335,026. Alternatively a combination of both of these methods may be used.
As the coating speed is increased, higher voltages are required to achieve the beneficial effects discussed above. However, as the voltage is increased, voltage induced perturbations are introduced to the bead, which can cause corresponding defects in the coating formed on the web. The severity of these voltage-induced defects increases as the applied voltage is increased. Therefore, there is a maximum practical voltage above which the disadvantageous voltage-induced defects outweigh the benefits obtained to the stability of the bead.
The threshold speed above which air entrainment occurs is also dependent on the surface roughness of the web to be coated. For example, International Patent Application No. WO 89/05477 discloses a bead coating example in which the threshold coating speed above which air entrainment occurs is slower for a rough web such as Matte paper than it is for a smoother web such as glossy paper. This is the case at low viscosity, such as 4.6 mPas and 6.5 mPas, even in the presence of an applied electrostatic field.
Apparatus and method of coating a web is required that enables the threshold coating speed above which air entrainment occurs to be maximised. In particular a method of coating a rough web is required which enables the web to be coated without air entrainment at speeds comparable to those at which smooth webs can be coated. The apparatus and method is required for use in the manufacture of, amongst others, photographic material such as films and photographic paper, magnetic recording tapes, adhesive tapes, etc.
According to the present invention, there is provided a method and apparatus of bead coating a web using a coating die. One or more layers of coating material is formed using the coating die, allowing the one or more layers to impinge on the web as the web and die move relative to each other. The web surface has an average peak to peak roughness (Rz) as defined by DIN 4768, ISO4287 or BS1134 greater than or equal to 2 xcexcm and less than or equal to 20 xcexcm, wherein the layer of coating material forming a wetting layer adjacent to the web has a viscosity of between 35 mPas and 200 mPas measured at a shear rate of substantially 10,000 sxe2x88x921. An electrostatic field is provided at the point at which the one or more layers impinge on the web to stabilize the one or more layers of coating material.
Preferably, the support is a backing roller and the electrostatic field is provided by a voltage of between 250V and 2000V between the one or more layers and the backing roller. More preferably, the voltage is between 600V and 1500V. Preferably, the roughness Rz is greater than 4 xcexcm and less than 10 xcexcm.
The present invention provides an apparatus and method whereby rough surfaces can be coated at high speed. The invention enables the web being coated to be conveyed at a speed greater than 400 cm/s relative to the coating die whilst avoiding the problem of air entrainment. Conventionally, it is suggested that to achieve higher coating speeds without air entrainment it is necessary to use smooth surfaced webs and a low viscosity wetting layer. In contrast to this, the present invention relies on the use of a wetting layer having a relatively high viscosity, a web to be coated having an average peak to peak roughness (Rz) of between 2 and 20 xcexcm and the provision of an electrostatic field to stabilise the one or more layers of coating material.
Since coating compositions of high viscosity are used, the invention therefore has the added advantage that the coating solutions on the slide of the coating die have improved stability. In addition, since the viscosity can be increased, the water content of the coating solutions can be reduced to better utilize the drying capacity of the manufacturing process. It can be seen that in the present invention, the use of high viscosity coating solutions does not compromise the maximum coating speed defined by air-entrainment.